The Unit's continuing focus on the vocal behavior of nonhuman primates this year concentrated on early vocal development, and the use of vocal interaction in social groups of adults. Infant vocalizing (in the first year of life) varies greatly from that of adults. Calling is more frequent and the mix of calls used is very different. Most important are the gradual structural changes evident in at least one vocalization, the "chuck" call, which we have previously identified as being an affiliative call with many language-like properties. Plasticity in vocal production is not typical of nonhuman primate vocal ontogeny, as it is in humans, and structural changes in the chuck call may be the first evidence of a role for experience or learning in a monkey call. Additional evidence of plasticity comes from our latest study of chuck usage in adult squirrel monkeys. Using a group of adult females, we broadcast to them the recorded calls of younger adult females who were born into the group but who had been removed from it before their distinctive chuck calls had matured. The older animals showed recognition of the younger females' chucks despite never having heard them before. Structural similarities between the chucks of the older and younger females provide a basis for recognition, but the origin of these (or other, undetected) commonalities is unclear. Daughters' chucks did not resemble mothers' more than others', and mothers were not more responsive to their own daughters. This suggests that common properties in the chuck calls of animals sharing genes or rearing environment ("familiarity" that is critical in eliciting a response from group members) may have a learned component. Chucks are extremely versatile calls, having both individually distinct, and group- identifying characteristics, in addition to flexible usage based on context. Certainly, the developmental life history of squirrel monkeys provides the opportunity for learning, as youngsters are exposed to millions of chuck calls from mothers and allomothers. There is no doubt that learning, if it figures at all in the production of nonhuman primate calls, has nowhere near its importance in human development. Nonetheless, by identifying points where learning occurs, we may gain insight into the kinds of activities where learned elements may have been most beneficial, and appeared earliest, in our ancestors.